An electrical power transmission network delivering electricity to consumers has to be able to handle voltage unbalances or instabilities, voltage sags, poor power factor, distortion or flicker occurring within the network. Reactive power control is one way to resolve such problems.
A STATCOM (STATic COMpensator) is an electrical device, which is based on voltage source converter (VSC) technology, and which can be used to provide reactive power support to the power transmission network. It is capable of producing or absorbing reactive power and can be adjusted by a high-speed control system.
FIG. 1 illustrates a basic STATCOM arrangement. In a basic configuration, the STATCOM 1 is made up of a DC voltage source 2, a DC/AC inverter 3 (voltage source converter, VSC) and transformer 4. Regulating the amplitude of the STATCOM output voltage controls the reactive power exchange of the STATCOM with a power network 5. If the amplitude of the output voltage exceeds the amplitude of the power network voltage, the reactive current flows through the transformer reactance from the STATCOM 1 to the power network 5 and the device generates reactive power. If the amplitude of the STATCOM output voltage is decreased to a level below that of the power network, then the current flows from the power network 5 to the STATCOM, which then absorbs reactive power. If the STATCOM output voltage is equal to the power network voltage, the reactive current is zero and the STATCOM does not generate nor absorb reactive power. The current drawn from the STATCOM is 90° shifted with respect to the power network voltage and it can be leading, i.e. generating reactive power, or it can be lagging, i.e. absorbing reactive power. Equivalently, leading (capacitive) or lagging (inductive) VARs [Volt-Ampere reactive] are produced.
The STATCOM comprises a main circuit, the voltage source converter VSC, that is designed to handle the injection or absorption of a certain amount of reactive power (“the rated power”). The main circuit may, for example, comprise insulated gate bipolar transistor (IGBT) devices, gate-turn-off thyristor (GTO) devices or integrated gate commutated thyristor (IGCT) devices.
There are situations in which it is advantageous to provide the STATCOM with an energy source on its DC side in order to provide some real power, also denoted active power, in addition to the reactive power generated to the network. That is, it is sometimes advantageous to be able to control not only the reactive power, but also to inject or absorb real power. For example, the real power can be utilized either as a source of reserve power when an energy deficit suddenly occurs within the network, or as a control power for managing transients and electromechanical oscillations in the network.
FIG. 2 illustrates a STATCOM having an energy source 6 (Ues) connected to its DC side. The energy source 6 may be materialized as an energy storage device that can temporarily supply energy that has previously been stored or as an energy supply that comprises some kind of conversion of non-electrical energy into electrical power. The energy source 6 may for example comprise conventional DC capacitors, super capacitors, electrochemical batteries, fuel cells or photovoltaric modules.
The energy sources 6 are adapted to a respective typical discharge cycle time, acting for seconds (conventional capacitors), minutes (super capacitors) or up to 30 minutes (batteries) or even continuously (fuel-cells or photovoltaic modules) depending on the type of storage element and loading conditions. Irrespective of type of energy source, in the following energy storage device 6, that is connected to the STATCOM DC link, they have in common that their DC voltage changes during the charge/discharge cycle. However, the DC voltage on the STATCOM must exceed a certain minimum level in order to make the STATCOM capable of providing the reactive power that it is rated for. In particular, the STATCOM has to be able to provide its rated reactive power even when the energy source is discharged or reaches its lowest accepted charge level.
As the DC voltage of the energy source 6 is connected directly to the DC-bus of the STATCOM, the latter must be designed to be able to operate with a varying DC voltage. The rated DC voltage, UDC, for the STATCOM cannot be higher than the lowest operating voltage of the energy source, i.e. UDC≦Ues, min=Ues(discharged). The STATCOM must be able to operate with all DC voltage levels up to the highest DC voltage of the energy source, i.e. up to Ues, max=Ues(fully charged).
The main circuit of the STATCOM has to be designed to handle the maximum DC voltage level Ues, max(fully charged) in order to cope with the variation of the DC voltage in the energy storage device 6. This is very costly, due to the expensive components that have to be used for such over-dimensioning. Typically, the DC voltage variation of the energy storage device 6 is 20-100% of the rated DC voltage for the STATCOM. The STATCOM is rated for handling a certain reactive power, e.g. 100 MVAr, and if this rated reactive power is high compared to the rated active power of the energy storage device 6, i.e. compared to the real power component, the costs caused by the varying DC voltage level are high.
FIG. 3 illustrates a prior art solution for handling the varying DC voltage of an energy storage device. In particular, a DC-to-DC converter 7 may be utilized for converting the voltage to a desired voltage. However, as DC/DC converters are quite complicated and costly, specifically as the whole active power has to pass through the DC/DC converter and because the voltage level that is suitable for the VSC may be quite high, the costs are increased substantially.
The costs for over-dimensioning the DC handling capability of a STATCOM in order for the STATCOM to be able to handle the varying DC voltage of a connected energy storage is thus very costly.